1. Field of the Invention
This invention relates to an interference measuring apparatus and a grating interference-type encoder for generating a phase difference signal and highly accurately measuring a length and an angle in an industrial apparatus such as FA (factory automation).
2. Related Background Art
A laser interferometer or an incremental encoder has heretofore been utilized as a highly accurate positioning apparatus such as FA. These apparatuses convert positional deviation into a pulse train, and count the pulse number at this time to thereby detect relative positional deviation. At this time, it is also necessary to detect a direction of movement and therefore, usually two or more phase difference signals A phase and B phase are outputted and phase shift of 90° is given with a sine wave signal of a cycle being 360°.
In such a high resolving power incremental encoder and laser interferometer, there is known a method of disposing two detecting optical systems with their spatial positions deviated from each other to thereby generate phase difference signals of A phase and B phase. There is also known a method of causing polarized light beams orthogonal to each other to interfere with each other through a quarter wavelength plate, converting them into a linearly polarized light beam of which the polarization direction rotates correspondingly to the phase difference between the wave fronts of the two light beams, and then further dividing it into a plurality of light beams. Those light beams are caused to be transmitted through polarizing plates disposed with their polarization axes deviated in different directions to thereby generate a phase difference signal light beam.
FIG. 1 of the accompanying drawings shows a perspective view of a non-contact distance sensor of the conventional laser interference-type, and a laser beam L from a coherent light source 1 passes through a collimator lens 2 and a non-polarizing beam splitter 3 and is polarized on the polarizing surface 4a of a probe-like polarizing prism 4. S-polarized light reflected by the polarizing surface 4a emerges from the probe-like polarizing prism 4 toward a slider 5, is reflected by the surface 5a to be measured by the slider 5, and again returns along the original optical path to the polarizing surface 4a of the probe-like polarizing prism 4.
On the other hand, P-polarized light transmitted through the polarizing surface 4a is reflected by the upper reference mirror surface 4b of the probe-like polarizing prism 4 and likewise returns to the polarizing surface 4a. These two polarized lights are re-combined on the polarizing surface 4a, travel through the probe-like polarizing prism 4, are reflected by a non-polarizing beam splitter 3, pass through a quarter wavelength plate 6 and an aperture in an aperture plate 7, and are amplitude-divided by a four-division diffraction grating 8. These amplitude-divided light beams pass through polarizing plates 9a–9d, and are received by the four areas 10a–10d of a light-receiving element 10. The minute displacement of the slider 5 is measured by an interference signal at this time.
However, in the above-described example of the prior art, the phase shift is given by arrangement or the like of the polarizing plates 9a–9d and therefore, there is a possibility that if there are an alignment error and manufacturing errors of the polarizing plates 9a–9d, the phase difference signal is not stable. On the other hand, in the case of the interference between linearly polarized lights orthogonal to each other, a space is required for arrangement of optical parts such as the quarter wavelength plate 6 and the four polarizing plates 9a–9d and therefore, the apparatus becomes bulky and the assembly adjustment of all these is necessary.